Conventional fast pyrolysis of biomass entails rapid heating of a biomass feedstock in a hypoxic environment to produce a combination of non-condensable gases (C1-C4), condensable pyrolysis vapors and solid carbonaceous char. Condensation of the pyrolysis vapors produces a pyrolysis oil that is thermally unstable at temperatures relevant for traditional refinery process, which makes it use as a refinery feedstock potentially problematic. Pyrolysis oil instability is largely due to a high concentration of oxygenates as well as reactive functional groups that are predisposed to form polymers such as oligosaccharides and phenolic resins. The presence of particulates (such as, for example char) comprising adsorbed metals may also contribute to instability.
Conventional catalytic pyrolysis (either in-situ or ex-situ) processes can produce partially deoxygenated products that are more easily upgraded. However, these processes utilize conditions and catalysts that favor non-selective deoxygenation (cracking of both C—O and C—C bonds), leading to coke formation on the upgrading/hydrostabilizing catalyst. Consequently, these processes suffer from low organic liquid yield and short catalyst life, and typically require a regenerator to oxidize the coke and restore catalytic activity. Further, due to the relatively low pressure environment and nonselective chemistry, the pyrolysis vapor or condensed pyrolysis oil products of these processes are not fully stable and require additional upgrading/hydrostabilizing (i.e., hydrotreatment) to decrease reactivity of remaining reactive functional groups within the products. This has proven to be a major barrier to commercial implementation of pyrolysis technology.
Conventional non-catalytic fast pyrolysis of biomass typically mixes biomass with a heated solid particles (i.e., “heat carrier”) to facilitate rapid heating of the biomass to a temperature ranging from 315° C. to 600° C. The resulting thermal-cracking of the heated feedstock produces non-condensable light gases, a solid carbonaceous char, and condensable pyrolysis vapors that can be converted to biofuels, or a component thereof.
Certain conventional processes for the pyrolysis of biomass upgrade pyrolysis vapors with a fixed bed hydrotreating reactor. Petroleum hydroprocessing technologies mainly use fixed bed reactors or, in few cases, ebulliated bed reactors. These reactors are good for petroleum feedstocks that normally contain less than 10 wt % of heteroatoms (O, S, N, etc.). However, the pyrolysis intermediates from biomass contain more than 30 wt % of oxygen and a large percentage of molecules containing unsaturated C—C bonds. Removing this oxygen and saturating these C—C double bonds releases much more heat that is extremely difficult to dissipate from a fixed bed reactor. Consequent over-reacting and local over-heating rapidly deactivates the beginning portion of the fixed bed reactor and eventually plugs the reactor. In addition, many conventional biomass pyrolysis methods inefficiently utilize multiple stages of hydrotreatment to convert the pyrolysis vapors to a stable liquid hydrocarbon product.
Certainly, there is a need to improve fast pyrolysis technology to facilitate the commercial-scale catalytic upgrading/hydrostabilizing of biomass-derived primary pyrolysis vapors into products that are fungible with current petroleum-derived liquid hydrocarbon fuels, or a component thereof.